The present invention relates generally to optical inspection systems, and more specifically to illumination techniques for optical inspection systems.
Optical inspection systems are commonly used in semiconductor manufacturing processes for quality control purposes. Specifically, optical inspection systems are used to detect and locate manufacturing defects. Optical inspection systems use an illumination source to illuminate the specimen being inspected. Various characteristics of the illumination source affect the resolution of the inspection system and the types of defects that can be detected. For instance, the intensity of the light produced from the light source affects the speed of the optical examination, and the wavelength of the light affects the types of defects that can be detected. Typically, more defect types and smaller defects can be detected when inspection is performed using shorter wavelength light because resolution is greater. Naturally, many inspection systems are designed to have high intensity and short effective wavelength illumination. Short effective wavelength illumination refers to an illumination source that produces light over a range of wavelengths such that the average wavelength is relatively short.
One type of illumination source used by optical inspection systems is an arc lamp. Arc lamps are broadband illumination sources in that they produce light in a broad range of wavelengths. A common arc lamp, for example, a mercury (Hg) arc lamp can produce light in the range of 200-600 nanometers (nm). Broadband illumination sources are advantageous since material contrast and scattering is affected by wavelength, and these mechanisms are important in maximizing the signal-to-noise ratio for detecting a variety of features and/or defects. They are also advantageous because the diversity of wavelengths in the illumination mitigates thin-film interference effects due to process variation produced thickness variations in transparent films (this interference is a noise source because it is generally not well-controlled on wafers). However, the intensity value within the range of wavelengths can vary among various sub-ranges of wavelengths because the intensity of light produced by arc lamps vary greatly from wavelength to wavelength. For example, Hg arc lamps produce light having spikes of high intensity at certain wavelengths while the intensity between such spikes is generally much lower. Also, these high intensity spikes tend to be located near the mid to longer wavelength end of the operational range of Hg arc lamps. As a result, optical inspection systems utilizing Hg arc lamps tend to have lower intensity in the shorter wavelength ranges, for example, in the wavelength range below approximately 300 nm. Practically, this means that the amount and quantity of information available from optical inspection systems can be limited by the characteristics of the illumination source. In light of the foregoing, optical inspection systems with increased resolution capabilities would be desirable.
The present invention pertains to techniques for increasing the intensity and resolution of optical inspection systems. These techniques involve combining the light beams from two separate illumination sources within a single optical inspection system. One implementation of the invention involves the utilization of a broadband illumination source and a narrowband illumination source. In such an implementation, one of the illumination sources can compensate the other illumination source in the wavelength range where illumination light intensity is low.
The technique of combining illumination light sources has many advantages aside from increasing the resolution. For instance, the speed of inspection devices can be increased because the overall available optical power is increased. Also, the equivalent spectral range where the inspection system operates can be relocated. In other words, the effective wavelength of the illumination source can be shortened in order to increase the resolution of the inspection system. Additionally, the operational spectrum of wavelengths used for inspection can be broadened. Broadband illumination sources are advantageous in that interference effects are minimized, maximum material contrast is provided due to either thin-film interference or scattering, and the amount of xe2x80x9ccolorxe2x80x9d noise in the inspection system is reduced. Furthermore, the combination of a short wavelength narrowband source with a conventional broadband source more closely approximates an ideal illumination source, which is both broadband and has high intensities in the shorter wavelengths.
These and other features and advantages of the present invention will be presented in more detail in the following specification of the invention and the accompanying figures, which illustrate by way of example the principles of the invention.